Summon the beast, then kill it with a destruction spell. You will probably not want to use a rune to fight them, as a guard/citizen may wander over it.

All spells needed can be purchased from [[Farengar Secret-Fire] in the Dragonsreach in Whiterun for 200-300 gold each.

Note that summoning a Dremora Lord is not recommended considering it cosst a whole 316 Magic and it is an Expert level spell, and you will probably only be able to summon it near end-game, when your destruction spells are more then likely strong enough.